1. Field of the Invention
The present invention relates to the field of artificial intraocular lenses suitable for replacement of the natural crystalline lenses in human eyes by means of surgical implantation. More particularly, this invention relates to intraocular lenses made from hydrogels which are shrunk temporarily prior to surgical implantation, thereby allowing a smaller incision to be utilized.
2. Discussion of Related Art
There have been many advances in surgical techniques and lens designs since the first implantation of an intraocular lens made from polymethylmethacrylate (PMMA) in 1949. Two of the recent, major advances are the use of phacoemulsification as a surgical technique for removing the natural crystalline lens from the eye and the introduction of materials other than PMMA for use in making intraocular lenses.
Phacoemulsification is an extracapsular technique that uses ultrasonic energy to fragment the natural crystalline lens into small particles. These small pieces are then removed by suction. One of the major advantages of this technique is that a small incision of only about 3 mm is required to remove the natural crystalline lens from the eye. This compares to incisions on the order of 11 mm when other surgical techniques are employed. When traditional, hard intraocular lenses made from PMMA are implanted, the incision must then be opened up to at least about 7 mm to allow insertion of the intraocular lens. This is viewed as a major drawback associated with the use of PMMA intraocular lenses, since surgeons would prefer to maintain the incision as small as possible.
There are other disadvantages associated with the use of PMMA in intraocular lenses. For example, such intraocular lenses cannot be autoclaved, but rather are usually made sterile by treatment with ethylene oxide which is a much less preferable treatment than autoclaving. Furthermore, there is increasing evidence that intraocular lenses made from PMMA can cause irreversible damage to endothelial cells. The above-discussed problems in connection with the use of PMMA in intraocular lenses have lead to the search for new materials which are suitable for use in intraocular lenses.
Hydrogels are ideal candidates for use in intraocular lenses since these materials have excellent biocompatibility and transparency, are autoclavable, have suitable mechanical properties and adaptable processing characteristics, and are inherently soft. Furthermore, it has been demonstrated that a hydrogel will cause very little endothelial cell damage, especially when compared to PMMA (Barrett, et al., American Journal of Ophthalmology, Vol. 98, pages 157-165 (1984)). Intraocular lenses made of a hydrogel have been described by Barrett in EP No. 136,807, by Wichterle in EP No. 166,051, by Koeniger in U.S. Pat. No. 4,449,257, by Mazzocco in U.S. Pat. No. 4,573,998, and by Blumenthal, et al., in Cornea, Vol. 1, pages 129-132 (1982). Typically these are hydrogels based on those described by Wichterle, et al., in U.S. Pat. No. 3,220,960. A comprehensive discussion of hydrogel intraocular lenses, including a discussion of the advantages of these lenses, is presented in the following text: Soft Implant Lenses In Cataract Surgery, Epstein, et al., Slack Incorporated, Thorofare, N.J. (1986). Reference is made to the above-cited publications for further background regarding the use of hydrogels in intraocular lenses. The contents of these publications are expressly incorporated herein by reference.
Hydrogels have several advantages when compared to polymethylmethacrylate as an intraocular lens material. The principal advantages are an ability to be autoclaved, a very significant reduction in endothelial cell damage and an ability to be brought to a higher level of purity. Other potential advantages include better biocompatibility and permeability, glare reduction, and physical adaptability to the internal environment of the human eye.
A further, very significant advantage associated with the use of hydrogels to make intraocular lenses is the flexibility and resilience of these materials. This flexibility and resilience provides the intraocular lenses with a capability to be inserted through a smaller incision than a polymethylmethacrylate intraocular lens by means of folding or other forms of manipulation, as described, for example, in U.S. Pat. No. 4,573,998, issued to Mazzocco. Another manner in which the size of a hydrogel lens can be reduced is by dehydrating the lens by some means and then allowing the lens to expand, in situ, via rehydration by the aqueous humor normally present in the eye. The latter method of reducing the size of the hydrogel lens can be accomplished by dehydrating the hydrogel and thereby reducing its size, inserting the intraocular lens in its reduced dimensions through a small incision and allowing the hydrogel intraocular lens to expand, in situ, to its normal equilibrium state via rehydration by the aqueous humor. This technique is based on the principle that hydrogels will take up water to an equilibrium point.
The dehydration/rehydration technique is proposed by Siepser in U.S. Pat. No. 4,556,998, as an improved method for the surgical implantation of hydrogel intraocular lenses, and is also mentioned in U.S. Pat. No. 4,573,998 (Mazzocco). Siepser describes the implantation of a hydrogel intraocular lens in its dehydrated state and its subsequent expansion via hydration by the natural fluid present in the eye; related approaches are described in U.S. Pat. No. 4,449,257 (Koeniger), and by Siepser and Epstein in respective chapters of the above-cited text titled "Soft Implant Lenses In Cataract Surgery". The approach described by Epstein concerns the insertion of hydrogel lenses which are either dehydrated, or partially hydrated by wetting a portion of the lenses prior to insertion. Epstein indicates that insertion of dry hydrogels can cause damage to the endothelium.
In the sole Example of U.S. Pat. No. 4,556,998, Siepser describes the implantation of an intraocular lens manufactured from a commercially available, dehydrated hydrogel that is sold under the tradename of HYDRON (manufactured by International Hydron). Siepser also describes the preparation of a hydrogel intraocular lens and subsequent dehydration of this lens via a freeze drying technique. Both of these approaches have a major disadvantage: dehydrated hydrogels cannot be sterilized by means of autoclaving. Since the autoclavability of hydrogels is a principal advantage associated with the use of hydrogels in intraocular lenses, this is a very serious disadvantage. Another disadvantage is that the implantation of dehydrated hydrogel lenses may also result in damage to the corneal endothelium, if there is contact between the lens and the endothelium. A still further disadvantage of implanting dehydrated hydrogel lenses is that a significant amount of time is required for the lenses to become hydrated following implantation. The Siepser patent indicates that from about 1 to about 24 hours or longer is required for full hydration. This is a serious disadvantage, since it frequently will not be possible to determine if a lens is properly positioned until it is fully hydrated and consequently fully expanded. Thus, it may be necessary to prolong a surgical procedure until a dehydrated lens becomes fully hydrated in situ.
It can thus be seen that while hydrogels are very viable candidates for use as an intraocular lens material, there is significant room for improvement in their surgical use as a replacement for the natural crystalline lens of the human eye. There is a real and continuing need for a method that will utilize the advantageous properties of hydrogels in a manner such that an intraocular lens can be delivered to the surgeon in a sterile state achieved by autoclaving, be inserted through a small incision, and swell rapidly following implantation. A primary object of this invention is to fill this need.
It is a more specific objective of this invention to treat a hydrogel intraocular lens in such a fashion as to maintain the lens in a state whereby it can be inserted through a small incision and rapidly expand when brought into contact with the natural fluids of the eye.
It is another specific objective of this invention to present to the ophthalmic surgeon a hydrogel intraocular lens that is capable of expanding rapidly after insertion and is available in a sterile state achieved by autoclaving.
The foregoing objectives and other general objectives of the present invention are achieved by deswelling a hydrated, hydrogel intraocular lens in a physiologically acceptable, hyperosmotic solution, autoclaving the deswelled lens and storing the deswelled, sterilized lens in the hyperosmotic solution.
The present invention will be more completely understood following review of the detailed description of the invention which follows.